Osteoarthritis (OA) is the most common rheumatic disease of the elderly. As part of the ongoing studies of OA in the Baltimore Longitudinal Study of Aging (BLSA) we have published data on radiographic hand OA in men and women, and have evaluated the association of metabolic and physiologic factors with the presence and distribution of hand and knee OA in both sexes, and with the progression of hand OA in women. In 182 women with longitudinal hand xrays, hand OA progresses more rapidly with increasing age. After age adjustment, body composition and bone mass are not independent predictors of hand OA. We developed a reliable atlas of knee OA for use in both clinical and epidemiologic studies. Using this atlas, we found that the prevalence of knee OA in 547 men and 351 women increases with age in both sexes, almost 50% of subjects aged equal to or greater than 70 have definite knee OA, and bilateral disease accounts for close to 50% of cases in both sexes aged equal to or greater than 60. In 465 men and 275 women aged equal to or greater than 40 years, after adjustment for age, obesity was significantly associated with the presence of knee OA in both sexes, while bone density was not. Serum levels of Insulin-like growth factor-I and urinary excretion of pyridinoline cross-links were inversely related to knee OA grade in 158 men and 100 women aged 19-86 years, but were no longer associated with OA after adjustment for age. In 431 men and 273 women aged equal to or greater than 40 years, after adjustment for age, we find a significant association in both sexes between hand and knee OA, supporting the concept of polyarticular OA.